Silver is chemically the most reactive element among the noble metals and tarnishes readily on exposure to sulfur bearing atmospheres. Because of its electronic state, silver exhibits a drastically different chemical behavior than, for instance, copper, although both metals are in the same group of the Periodic Table. Thus, silver tarnishing is quite different from corrosion of other metals.
Tarnishing, sometimes referred to as discoloration, is caused by a silver oxidation process in which sulfide is formed. Food such as onions, mustard and eggs which contain organic sulfur compounds are also known to tarnish silver. See Singh et al., "Silver Tarnishing and its Prevention--A Review" Anti-corrosion Methods and Materials, v. 30 (July 1983) pp. 4-8.
Silver tarnishing is also known to occur when a bleaching agent used in detergent compositions oxidizes the silver to silver oxide. This oxidation process causes surface blackening of the silver leaving undesirable tarnishing of silverware when machine dishwashed.
Use of organic compounds to enhance the resistance of a silver surface to tarnishing has been described in Singh et al., supra pp. 5-6. Certain triazoles, particularly benzotriazole and its variations have been described as useful for silver inhibiting agents in detergent compositions. See CH 673033 (1990) and U.S. Pat. No. 4,321,166 (Procter & Gamble--1982).
U.S. Pat. Nos. 2,618,606 (Procter & Gamble) and 2,618,608 (Procter & Gamble) disclose the use of azoles, including 1,2,3-triazole, imidazole and pyrazole as discoloration inhibitors used in detergent compositions for nonferrous copper and brass-like metals. The described detergents do not, however, contain a bleaching agent which complicates silver tarnish inhibition. The patents further state that a mixture of copper inhibitors are necessary to prevent tarnishing over a broad pH range.
Moreover, detergent compositions are increasingly being based on peroxygen bleaching agents and are being formulated to be milder to produce more environmentally friendly products. The problem of tarnishing of silver and silver plated articles has thus become more severe.